Satellite images of 30 October 2008 show the development
over north-central Italy of rainbands and multiple
waves during a strong south-westerly wind episode
associated with a deepening synoptic trough and
cold front passage.
The event was studied by means of the ISAC model chain constituted
of the hydrostatic model BOLAM and the nested non-hydrostatic model
MOLOCH at 1.1 km resolution.
Diagnostics of model output was performed to reveal the
physical origin of the dynamical features and precipitation
field as simulated.
Based on our results we propose a theoretical framework in
which symmetric instability underlies some of the observed
precipitation patterns

P. Malguzzi

M. Fantini

A. Buzzi

Almgren, Torgny

Name not available

Time planning under uncertainty in a mining environment

Abstract. Satellite images of 30 October 2008 show the development over north-central Italy of rainbands and multiple waves during a strong south-westerly wind episode associated with a deepening synoptic trough and cold front passage. The event was studied by means of the ISAC model chain constituted of the hydrostatic model BOLAM and the nested non-hydrostatic model MOLOCH at 1.1 km resolution. Diagnostics of model output was performed to reveal the physical origin of the dynamical features and precipitation field as simulated. Based on our results we propose a theoretical framework in which symmetric instability underlies some of the observed precipitation patterns.
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Numerical study of a banded precipitation event over Italy

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Adv. Geosci., 26, 19–23, 2010www.adv-geosci.net/26/19/2010/doi:10.5194/adgeo-26-19-2010© Author(s) 2010. CC Attribution 3.0 License.Advances inGeosciencesNumerical study of a banded precipitation event over ItalyP. Malguzzi, M. Fantini, and A. BuzziISAC-CNR, via Gobetti 101, 40129 Bologna, ItalyReceived: 15 February 2010 – Revised: 24 May 2010 – Accepted: 25 May 2010 – Published: 30 June 2010Abstract. Satellite images of 30 October 2008 show the de-velopment over north-central Italy of rainbands and multiplewaves during a strong south-westerly wind episode associ-ated with a deepening synoptic trough and cold front pas-sage. The event was studied by means of the ISAC modelchain constituted of the hydrostatic model BOLAM and thenested non-hydrostatic model MOLOCH at 1.1km resolu-tion. Diagnostics of model output was performed to revealthe physical origin of the dynamical features and precipita-tion ﬁeld as simulated. Based on our results we propose atheoretical framework in which symmetric instability under-lies some of the observed precipitation patterns.1 IntroductionMSG Satellite images of 30 October 2008 (Fig. 1) show thedevelopment over North-Central Italy of several rainbandsand multiple wavetrains during a strong south-westerly windepisodeassociatedwithadeepeningsynoptictroughandcoldfront passage. The nearly two-dimensional structure of thelarge scale ﬂow (Fig. 2) and the ubiquitous orographic forc-ing makes this a nearly ideal setup to test for the occurrenceof slanted circulations developing in a symmetrically unsta-ble environment.For this purpose we used the ISAC model chain, con-stituted of the hydrostatic model BOLAM and the non-hydrostatic model MOLOCH, to simulate the episode, andperformed diagnostics that support the hypothesis that sym-metric instability was present during the event.Correspondence to: M. Fantini(m.fantini@isac.cnr.it)2 The non-hydrostatic model MOLOCHThe non-hydrostatic model MOLOCH integrates the fullycompressible set of equations, using as prognostic variablespressure, temperature, speciﬁc humidity, horizontal and ver-tical velocity components, and ﬁve condensed water species.Model dynamics are integrated in time with an implicitscheme for the vertical propagation of sound waves, whileexplicit, time-split schemes are implemented for the remain-ing terms. Advection is computed using the Eulerian WAFscheme (Billet and Toro, 1997). The vertical grid is stretchedexponentially with height. The microphysical scheme isbased on the parameterization proposed by Drofa and Mal-guzzi (2004). The physical processes determining the timetendency of speciﬁc humidity, cloud water/ice and precipi-tating water/ice are divided into “fast” and “slow” ones. Fastprocesses involve transformations between speciﬁc humidityand cloud quantities, while slow ones involve rain/snow/hailproduction and fall. Temperature is updated by imposing ex-actenthalpyconservationatconstantpressure. Fallofprecip-itation is computed with the stable and dispersive backward-upstream scheme, with speed depending on concentration.For a full presentation of the model see Malguzzi et al.(2006), which also contains a description of BOLAM.3 SimulationBOLAM was run for 15h, starting from NCEP analysis at00:00UT with boundary conditions every three hours takenfrom the NCEP global daily forecast, with horizontal res-olution 13km and 40 vertical levels. The non-hydrostaticMOLOCH was then nested in the BOLAM domain, start-ing at 01:00UT, with boundary conditions updated at 30mininterval. MOLOCH was run at 0.01 degrees resolution, cor-responding to 1.1km, and 50 vertical levels, on the domainshown in Fig. 3.Published by Copernicus Publications on behalf of the European Geosciences Union.20 P. Malguzzi et al.: Banded precipitation eventFig. 1. Satellite image at 10:30UT 30 October 2008 showing wavetrains in the lee of the Northern Apennines mountains and sev-eral rainbands at various stages of development. Image Copyright©2008 EUMETSAT, courtesy V. Levizzani, ISAC-CNR.Fig. 2. 500 hPa analysis (height and temperature) at 12:00 UTC 30October 2008. Modiﬁed from NWS DIFAX Weather Map Archiveat http://archive.atmos.colostate.edu/.The MOLOCH simulation generates deep convective ac-tivity from early morning to about 12:00UTC, mainly in thesouth-east part of the domain, in the warm and moist sectorahead of the front.Between 09:00 and 10:30 slanted convective lines ap-pear over Central Italy, oriented SW to NE. These lines arealigned with the jet, perpendicular to the Apennines and welldistinct from the lee waves that can be observed, in Fig. 1,Fig. 3. MOLOCH domain of integration, topography and land-seamask. Contour interval 250m. The thick straight lines identify thelocation of the cross-sections shown in Figs. 9–12.Fig. 4. Vertical velocity at 700 hPa, 09:30UTC from MOLOCHsimulation. Contour interval 0.5ms−1.over the Po Valley. From these simulations we show verticalvelocity w at 700hPa (Fig. 4), accumulated precipitation in30min (Fig. 5) and wind vectors at two levels (Figs. 6 and7). A strong downslope wind started early in the morning inthe lee of the Northern Apennines, reaching maximum am-plitude at 09:00UT, when the simulated 10m wind reachesmaxima of 30m/s. Damages due to strong gustiness were re-ported in several mountain locations during the late morningAdv. Geosci., 26, 19–23, 2010 www.adv-geosci.net/26/19/2010/P. Malguzzi et al.: Banded precipitation event 21Fig. 5. Accumulated precipitation between 09:00 and 09:30UTC.Contour interval 2.5mm, starting from 0.5 to remove the noisy zerocontour.Fig. 6. Wind vectors at 10 m at 09:30 UT, with contours of windspeed, interval 2ms−1.hours. The MOLOCH simulation reproduces well the inten-sity of this wind, as seen in Figs. 6 and 7.The waves over the Po Valley are orographically gener-ated, have maximum amplitude near the ground and displacetoo little moisture to form any precipitation. The cloud bandsover Central Italy are convectively active, with maximum in-tensity in the mid troposphere, and are precipitating. We alsoshow (Fig. 8) the simulated relative humidity at 600hPa thatFig. 7. Same as Fig. 6 except at 700 hPa. Contour interval 5ms−1.Fig. 8. Relative humidity at 600 hPa, 09:30UTC. Contour interval10 percent.exhibits a rather sharp boundary between the moist warmarea ahead of the front and the dryer air behind it.Figures 9, 10 and 11 present cross-sections of relative hu-midity (colours) and cloud condensate (water+ice, contours)taken along the sections identiﬁed by thick straight lines inFig. 3. Several episodes of slantwise convection at vari-ous stages of evolution can be seen, with cloud top reach-ing 7000m at most. Cells arising from more traditionalwww.adv-geosci.net/26/19/2010/ Adv. Geosci., 26, 19–23, 201022 P. Malguzzi et al.: Banded precipitation eventP. Malguzzi et al.: BANDED PRECIPITATION EVENT 5Fig. 9. Relative humidity (color) and water condensate (thick con-tours) at 09:30 UTC on section x1 (see Fig. 3)Fig. 10. Same as Fig. 9 on section y2 at 9:30 UT.Fig. 11. Same as Fig. 9 on section y1 at 10:30 UTC.Fig. 12. Saturated equivalent potential temperature θe (color) andpseudo angular momentum M =v+fx (thick contours) at 09:00UTC on section x2 of Fig. 3 (contour interval 10 m s−1, arbitraryorigin).Fig. 9. Relative humidity (color) and water condensate (thick con-tours) at 09:30UTC on section x1 (see Fig. 3).P. Malguzzi et al.: BANDED PRECIPITATION EVENT 5Fig. 9. Relative humidity (color) and water condensate (thick con-tours) at 09:30 UTC on section x1 (see Fig. 3)Fig. 10. Same as Fig. 9 on section y2 at 9:30 UT.Fig. 11. Same as Fig. 9 on section y1 at 10:30 UTC.Fig. 12. Saturated equivalent potential temperature θe (color) andpseudo angular momentum M =v+fx (thick contours) at 09:00UTC on section x2 of Fig. 3 (contour interval 10 m s−1, arbitraryorigin). Fig. 10. Same as Fig. 9 on section y2 at 09:30 UT.convective instability, due to moist unstable conditions atlow levels, are also present in the simulation. These cellstend to develop into mainly shallow convection, which co-exists with slantwise convection. The last ﬁgure (Fig. 12)shows a typical cross-section of saturated equivalent poten-tial temperature. Unstable conditions are indeed observedin the layer below 2000m. Solid contours in the same ﬁg-ure are pseudo angular momentum (M, see Appendix A)iso-surfaces computed with the wind across the section.Where slanted convection originates and develops, isolinesP. Malguzzi et al.: BANDED PRECIPITATION EVENT 5Fig. 9. Relative humidity (color) and water condensate (thick con-tours) at 09:30 UTC on section x1 (see Fig. 3)Fig. 10. Same as Fig. 9 on section y2 at 9:30 UT.Fig. 11. Same as Fig. 9 on section y1 at 10:30 UTC.Fig. 12. Saturated equivalent potential temperature θe (color) andpseudo angular momentum M =v+fx (thick contours) at 09:00UTC on section x2 of Fig. 3 (contour interval 10 m s−1, arbitraryorigin).Fig. 11. Same as Fig. 9 on section y1 at 10:30UTC.P. Malguzzi et al.: BANDED PRECIPITATION EVENT 5Fig. 9. Relative humidity (color) and water condensate (thick con-tours) at 09:30 UTC on section x1 (see Fig. 3)Fig. 10. Same as Fig. 9 on section y2 at 9:30 UT.Fig. 11. Same as Fig. 9 on section y1 at 10:30 UTC.Fig. 12. Saturated equivalent potential temperature θe (color) andpseudo angular momentum M =v+fx (thick contours) at 09:00UTC on section x2 of Fig. 3 (contour interval 10 m s−1, arbitraryorigin).Fig. 12. Saturated equivalent potential temperature θe (color)and pseudo angular momentum M = v +fx (thick contours) at09:00UTC on section x2 of Fig. 3 (contour interval 10ms−1, ar-bitrary origin).of equivalent potential temperature and M tend to be, onaverage, aligned, realizing the condition for marginal moistsymmetric instability.Adv. Geosci., 26, 19–23, 2010 www.adv-geosci.net/26/19/2010/P. Malguzzi et al.: Banded precipitation event 234 ConclusionsWe do not consider this evidence conclusive. However, thegrowth and permanence, for times of the order of one hour,of narrow regions of saturated, precipitating, slanted ascentacross the wind (NOT differential advection) and in a direc-tion consistent with the wedge of symmetric instability, andthe formation of persistent neutral channels as the end re-sult of the slanted ascent (slantwise convective adjustment)are strong indications that symmetric instability may havetaken place in this event. The coexistence of conditionsleading to moist convection and symmetric instability, aswell as the complexity introduced by orography, the three-dimensionality and non-stationarity of the large scale ﬂowmake it difﬁcult to isolate the signature of moist symmet-ric instability in this environment. Numerical simulations inmoreidealizedandcontrollableconditionsshouldbethenextstep in the process to understand the role played by symmet-ric instability in determining frontal structure.Appendix ASymmetric instabilityForanearlyparallelﬂowonecanlookatarepresentativesec-tion across the ﬂow and consider the two dimensional prob-lem. Deﬁne M =fx+v and let B =θ in dry conditions, orB =θe for saturated ﬂow. Then Mx <0 denotes inertial in-stability and Bz <0 denotes convective instability. When thederivatives are positive but the Jacobian Q is negative:Q=∂(M,B)∂(x,z)=MxBz−MzBx <0 (A1)then unstable slanted updrafts are still possible. As is thecase with ordinary convection, if the air is sub-saturated theinstability is “conditional”, i.e. external forcing is requiredto kick-start the motion. Moreover, the consideration of con-densate loading changes somewhat the above condition (Fan-tini and Malguzzi, 2008).This instability favors structures elongated along thewind and it is therefore generally invoked as a possibleexplanation for frontal rain-bands (Bennetts and Hoskins,1979). The quantity Q above (a two-dimensional potentialvorticity) can be interpreted as the gradient of M alongthe surfaces of constant B , or the gradient of B alongM surfaces (with appropriate changes of sign). The stateof neutrality Q = 0, easily identiﬁed as the two family ofisopleths of B and M become parallel, presumably as theﬁnal outcome of the instability, is usually referred to as“slantwise convective adjustment”.Edited by: G. BoniReviewed by: one anonymous refereeReferencesBennetts, D. A. and Hoskins, B. J.: Conditional Symmetric Insta-bility – A Possible Explanation for Frontal Rainbands, Q. J. Roy.Meteor. Soc., 105, 945–962, 1979.Billet, S. and Toro, E. F.: On WAF-type schemes for multidimen-sional hyperbolic conservation laws, J. Comput. Phys., 130, 1–24, 1997.Drofa, O. V. and Malguzzi, P.: Parameterisation of microphysicalprocesses in a non-hydrostatic prediction model, in: Proceed-ings of 14th Intern. Conf. on Clouds and Precipitation (ICCP),Bologna, Italy, 19–23 July 2004, 1297–1300, 2004.Fantini, M. and Malguzzi, P.: The slope of moist symmetric insta-bility with water loading, J. Atmos. Sci., 65, 2922–2935, 2008.Malguzzi, P., Grossi G., Buzzi A., Ranzi R. and Buizza R.:The 1966 ‘century’ ﬂood in Italy: A meteorological andhydrological revisitation, J. Geophys. Res., 111, D24106,doi:10.1029/2006JD007111, 2006.www.adv-geosci.net/26/19/2010/ Adv. Geosci., 26, 19–23, 2010

Numerical study of a banded precipitation event over Italy

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